<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title></title>
    <!-- 引入vue -->
    <script type="text/javascript" src="../js/vue.js"></script>
</head>

<body>

    <div id="root">
        <h2>今天天气很{{info}}</h2>
        <button @click="changeWeather">切换天气</button>

    </div>

</body>
<script type="text/javascript">
    Vue.config.productionTip = false //阻止vue在启动生成生产提示。

    const vm = new Vue({
        el: '#root',
        data: {
            isHot: true,

        },
        computed: {
            info() {
                return this.isHot ? "炎热" : "凉爽"
            }
        },
        methods: {
            changeWeather() {
                this.isHot = !this.isHot
            }
        },
        watch: {
            //正常写法
            // isHot:{
            //    //immediate:true, //初始化时让handler调用一下
            //    //handler什么时候调用？当isHot发行改变时
            //    handler(newValue,oldValue){
            //     console.log('isHot被改变了',newValue,oldValue);
            //    }
            // }

            //简写
            // isHot(newValue, oldValue) {
            //     console.log('isHot被改变了', newValue, oldValue);
            // }
        }
    })
    //完整写法
    // vm.$watch('isHot', {
    //     immediate: true, //初始化时让handler调用一下
    //     //handler什么时候调用？当isHot发行改变时
    //     handler(newValue, oldValue) {
    //         console.log('isHot被改变了', newValue, oldValue)
    //     }
    // })

    //简写
    vm.$watch('isHot', function (newValue, oldValue) {
        console.log('isHot被改变了', newValue, oldValue)
    })

</script>

</html>